The ShipStation integration provides an estimation of the shipping cost. However, it is possible to change the estimated shipping cost to the actual shipping cost before applying it.
You can do this after clicking to Approve a Sales Order - here's how:
Alternatively, you can do this in a similar popup when you click to Approve a Shipment.
If you need more information about how to integrate Megaventory with ShipStation, you can read about it here.
Do you need more assistance? Reach out to us!